Dipole locator using balanced antenna signals

ABSTRACT

An antenna arrangement. The arrangement uses four conductive loops, each within a distinct plane from the other conductive loops. The four conductive loops have a common center point. Each loop is within a dipole magnetic field, and detects a component thereof. By balancing the signals received between matched pairs of the conductive loops, the difference between the signals can be used to guide the antenna arrangement to a null point—that is—a point in the magnetic field where each pair of conductive loops is balanced. The antenna arrangement can further be used to determine the depth of the dipole field source using the magnitude of the field.

SUMMARY

The present invention is directed to an antenna having four conductiveloops, each of the four conductive loops situated within a single planeand having a center, in which the centers of the four loops coincide andwhich none of the planes of the four loops coincide.

The present invention is also directed to an above-ground antennaarrangement having first, second, third and fourth conductive loops. Thesecond conductive loop is surrounded by the first conductive loop. Thefourth conductive loop is surrounded by the third conductive loop. Eachof the first, second, third and fourth conductive loops are planar anddefine separate planes. Each of the first, second, third and fourthconductive loops have a common centerpoint.

The present invention is further directed to an antenna assemblycomprising a first, second, third and fourth conductive loop. Each loopis situated in a plane and defines a normal vector normal to the planeand extending through the centerpoint of the loop. The centerpoints ofthe first, second, third and fourth loops are coincident, and the first,second, third and fourth normal vectors are distinct.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a horizontal directional drilling systemfor drilling a horizontal borehole.

FIG. 2 is an illustration of a tracker utilizing an antenna arrangementto detect a below ground magnetic field source. The tracker is shown inphantom such that the antenna arrangement is visible.

FIG. 3 is a left side elevation view of an antenna arrangement of thepresent invention utilizing four printed circuit board antennas.

FIG. 4 is a left side elevation view of an alternative antennaarrangement utilizing four loop antennas of magnet wire.

FIG. 5 is a right side elevation view of the antenna arrangement of FIG.3.

FIG. 6 is a front elevation view of the antenna arrangement of FIG. 3.

FIG. 7 is a back elevation view of the antenna arrangement of FIG. 3.

FIG. 8 is a bottom plan view of the antenna arrangement of FIG. 3.

FIG. 9 is a top plan view of the antenna arrangement of FIG. 3.

FIG. 10 is a top front perspective view of the antenna arrangement ofFIG. 3.

FIG. 11 is a top front perspective view of the antenna arrangement ofFIG. 3. In FIG. 11, the support structure is removed and normal vectorsof the various antenna components are shown. The vector normal toantenna 30 is disposed into and out of the page.

FIG. 12A is a diagram showing the normal vectors of the antennacomponents disposed from the front, or as oriented in FIG. 6. The angleis slightly offset so that normal vectors 30 a and 32 a are distinct.

FIG. 12B is a diagram showing the normal vectors of the antennacomponents disposed from the right, or as oriented in FIG. 5. The angleis slightly offset so that normal vectors 34 a and 36 a are distinct.

FIG. 12C is a diagram showing the normal vectors of the antennacomponents disposed from the top, or as oriented in FIG. 9. The angle isslightly offset so that normal vectors 34 a and 36 a are distinct andnormal vectors 30 a and 32 a are distinct.

DETAILED DESCRIPTION

The present invention is directed to an antenna arrangement 10, shown inFIGS. 2-11, for use with a tracker 12 used in horizontal directionaldrilling operations. In FIG. 1, a horizontal directional drilling system14 is shown. The system 14 uses a drill rig 16 to advance a drill string18 carrying a downhole tool 20 underground to create a borehole 22. Abeacon 24 positioned within the downhole tool 20 emits a beacon signal.The tracker 12, positioned on or above the ground surface 26, detectsthe beacon signal and communicates the beacon's position to a trackeroperator 28. The tracker 12 detects the beacon signal using multipleantennas, as shown herein.

With reference to FIG. 2, a representative tracker 12 is shown.Typically, the antenna arrangement 10 is disposed on the tracker 12 at abottom end of its frame 29. A processor (not shown) is utilized in thetracker to interpret signals received by the antenna arrangement 10. Adisplay 31 may be disposed at the top end of the frame 29 to help theoperator 28 (FIG. 1) visualize information about the field generated atthe beacon 24. In addition or in the alternative, the tracker 12 maycommunicate this information with the drill 16. While one antennaarrangement 10 is disposed in the tracker 12 in FIG. 2, multiple antennaarrangements may be used together on a single tracker, either verticallyor horizontally displaced on the frame 29.

With reference to FIGS. 3-10, the antenna arrangement 10 comprises fourindividual antennas: a first antenna 30, a second antenna 32, a thirdantenna 34, and a fourth antenna 36. The antennas 30, 32, 34, and 36 areheld together in the antenna arrangement 10 via a support structure 38.The antennas 30, 32, 34, and 36 all have the same center point thatcorresponds with a center point 40 formed in the support structure 38,as shown in FIGS. 8-10. The support structure 38 may be formed ofplastic. In alternative embodiments, the support structure may be formedof a metal capable of shielding the antennas from electric fields. Inany case, each of the antennas 30, 32, 34, 36 is electricallyindependent from the others.

Each of the antennas 30, 32, 34, and 36 is a printed circuit board (PCB)antenna. Printed circuit board antennas are fabricated using microstrips(not shown) on a circuit board frame 60. Alternatively, as shown in FIG.4, each of the antennas 30, 32, 34, and 36 may be a coil 44 of magnetwire wound on a frame 41 having the cross-sectional shape of a hollowrectangle having radius corners. The frame 41 may be made of the samematerial as the support structure 38. The coil 44 of magnet wire, suchas litz wire, is positioned within an externally-facing groove 42. Eachcoil 44 may be wound in only one direction. The coils 44 may be coveredwith a copper spray or tape to help insulate the coils from electricfields.

PCB antennas, such as shown in FIGS. 3, and 5-11, have some advantageover coils 44 of magnet wire, as they may be made to precisionspecifications, without the need for the tolerances associated withwinding the magnet wire into coils 44. However, the geometries of theassemblies to shown in FIG. 3 and FIG. 4 are similar, and discussion ofthe PCB antennas below also applies to coil-type antennas.

In either configuration, the antenna may be referred to as conductiveloops, having an aperture defined within the conductive loop.

Continuing with FIGS. 3, and 5-11, the first antenna 30 circumvents thesecond antenna 32 such that the antennas are perpendicular to oneanother. The first antenna 30 overlaps the second antenna 32 at twofirst points 46, as shown in FIG. 3 and FIG. 5. The two first points 46are connected by a first reference line 100 a. (FIG. 6) The planes inwhich the first 30 and second 32 antennas are situated meet at referenceline 100 a. The first and second antennas 30 and 32 are held by thesupport structure 38 such that each antenna 30 and 32 sits at a 45degree angle with respect to a horizontal plane 48, as shown in FIG. 3.

The third antenna 34 circumvents the fourth antenna 36 such that theantennas 34 and 36 are perpendicular to one another. The third antenna34 overlaps the second antenna 36 at two second points 50, as shown inFIGS. 6-7. The two second points 50 are connected by a second referenceline 100 b (FIG. 8). The planes in which the third 34 and fourth 36antennas are situated meet at reference line 100 b. The third and fourthantennas 34 and 36 are held by the support structure 38 such that eachantenna 34 and 36 sits at a 45 degree angle with respect to thehorizontal plane 48, as shown in FIG. 6.

While the each antenna pair 30, 32 and 34, 36 is at a 45 degree anglewith respect to horizontal, the balancing aspect of the antennaarrangement 10 may be functional at alternate angles to the horizontalplane 48. Preferably, each antenna of a given pair are at the same anglerelative to the horizontal plane 48.

The third and fourth antennas 34 and 36 are positioned inside of thefirst and second antennas 30 and 32 such that the second antenna 32circumvents the third antenna 34. The first and second antennas 30 and32 have a larger aperture area than the third and fourth antennas 34 and36. As shown in FIG. 8, the first reference line 100 a and secondreference line 100 b are perpendicular. If reference lines 100 a, 100 bare perpendicular, and the antenna arrangement 10 is oriented such thatthe second reference line 100 b is parallel to the beacon length(roughly the orientation of the drill string 18), then the first 30 andsecond 32 antennas detect the vertical component of the field, and thecomponent of the field parallel to the beacon 24. The third 34 andfourth 36 antennas detect the vertical component of the field, and thecomponent of the field perpendicular to the beacon 24.

The antenna arrangement 10 described in the figures is different fromantenna arrangements known in the art that typically consist of threeseparate and orthogonal antennas oriented in Cartesian alignment (x, y,and z). That is, each of the three orthogonal antennas is disposed in aplane which sits at a ninety degree angle to each of the other two. Eachantenna of such arrangement detects the beacon signal on a differentaxis. The “z” axis corresponds to up-down direction of the beaconsignal, and the “x” and “y” axes correspond to the right-left,forward-backward direction of the beacon signal. Such an antennaarrangement is shown in U.S. Pat. No. 7,786,731, issued to Cole, et al.,the contents of which are incorporated herein by reference.

Such antenna arrangements detect a position of the beacon 24 by findingthe “null” points in the transmitted field. The “null” point occurs whenthe only component being read by the tracker 12 is the “z” axis or thevertical axis. Thus, when the tracker 12 is at the “null” point, theantenna detecting the “z” axis has a signal, but the antennas detectingthe “x” and “y” axes have no signal. Such signal readings indicate thatthe tracker 12 is in-line with the borepath, but either ahead of orbehind the beacon 24. Thus, the “null” points occur at two locations onthe beacon's transmitted field—one in front of the beacon 24 and onebehind.

Since noise is always present in the field, it can be difficult to getthe antenna components detecting the “x” and “y” axes to read “nosignal”. This difficulty leads to discrepancies and variation whentrying to find the “null” points. This situation also occurs directlyover the beacon 24. Because it is difficult to get the antennacomponents to ever receive “no signal”, the user must interpret the datato find the location of the beacon 24 and “null” points.

In the present embodiment, the antenna arrangement 10 detects theposition of the beacon 24 underground by digitally balancing the signalsreceived by each set of paired antennas—first and second antennas 30, 32and third and fourth antennas 34, 36. Digitally balancing the antennasignals means that when a given signal is generated completely in the“null” axis, all signals generated by the antennas will read the same tothe processor, and therefore the user. Thus, at a null, where previouslycomponents of the dipole field were merely parallel (and thereforetheoretically invisible) to two of the antennas, in the antennaarrangement 10 of the present invention, the third and fourth antennaswill detect equal signals, and the first and second antennas will detectequal signals.

Prior to operation, the antennas 30, 32, 34, and 36 may be digitallybalanced by placing the arrangement 10 in a fixture and driving a “null”field through them. The processor will read each antenna signal andcreate a balance matrix through which all future readings will bemultiplied through to normalize the antenna signals.

Rather than detecting the beacon signal on three axes (x, y, and z), theantennas 30, 32, 34, and 36 each detect, in pairs, a particular balanceof the magnetic field radiated from the beacon 24. The first and secondantennas 30 and 32 detect the forward and backward direction of thefield along the borepath, and the third and fourth antennas 34 and 36detect the side-to-side balance of the field at the centerpoint 40 ofthe antenna arrangement 10.

As shown in FIG. 11, each antenna 30, 32, 34, 36 has a distinctcorresponding normal vector 30 a, 32 a, 34 a, 36 a. In FIG. 11, axis 30a extends directly out of the page. Each normal vector 30 a-36 a isperpendicular to the aperture of its corresponding antenna 30-36, andpasses through the common centerpoint 40. FIGS. 12A, 12B, and 12C showthese normal vectors 30 a, 32 a, 34 a, 36 a without the structure of theantenna arrangement 10 from the front, left and top respectively. Whenthe paired antennas are orthogonal, as in FIGS. 3, 5-11, the normalvectors of each pair will be perpendicular.

While orthogonal antenna pairs are shown here, it should be understoodthat, so long as the angles between each antenna of a pair and thehorizontal plane 48 are equal and opposite, other orientations ofantenna pairs (that is, antennas 30, 32 on one hand, and antennas 34, 36on the other) may be used to balance the signals and perform thetracking operation disclosed herein.

The discrepancies in signal strength between the first and secondantennas 30 and 32 and between the third and fourth antennas 34 and 36help the operator find the “null” point along the borepath. Thus, thepairs of antennas 30 and 32 and 34 and 36 work to guide the operator 28in two dimensions to a position directly over the “null” point.

None of the antennas 30, 32, 34, and 36 directly detect the field on avertical or “z” axis. Instead, the position of the beacon along the “z”axis is determined by a mathematical calculation of the beacon 24location using signals detected by the antennas 30, 32, 34, and 36, eachof which detect a component of the vertical field.

In operation, the tracker operator 28 is instructed which direction tomove by commands displayed on the display 31 of the tracker 12. Tostart, the tracker operator 28 will turn on the tracker 12 and hold itshandle parallel to the direction of the drill string 18, which shouldapproximate the center-line of the transmitter on the beacon 24.

The display 31 will then provide the operator 28 with a direction tomove. For example, if the first antenna 30 has a different signal thanthe second antenna 32, the monitor will direct the operator 28 to movethe tracker 12 forwards or backwards until the two signal strengths arebalanced. Likewise, if the third antenna 34 has a different signal thanthe fourth antenna 36, the monitor will direct the operator 28 to movethe tracker 12 right or left until the two signal strengths arebalanced.

A similar situation is presented when the tracker 12 is positioneddirectly over the beacon 24. When perfectly over the beacon antenna, thesecond antenna 32 is balanced with the first antenna 30. If the operatormoves the tracker 12 forward, the second antenna 32 will have highersignal strength than the first antenna 30, indicating that the trackershould move backwards. The opposite scenario plays out if the tracker 12moves behind the beacon 24.

When the tracker 12 arrives at the “null” point, all four antennas 30,32, 34, and 36 are balanced. The tracker 12 notifies the operator 28 ofsuch arrival on the monitor. The operator 28 then marks position of thefirst “null” point found and attempts to find the second “null” pointusing the same balancing method. Once the operator 28 has found both“null” points, the operator will walk the line between the null pointsuntil the first and second antennas 30 and 32 are again balanced. Atthis point, the tracker 12 will indicate to the operator 28 that thetracker 12 is positioned directly over the beacon 24. The total fieldwill then be calculated to determine the depth of the beacon 24.

The total field may be calculated using the mathematical formulas andmethods described in U.S. Pat. No. 9,329,297, titled Dipole LocatorUsing Multiple Measurement Points, issued to Cole, the entire contentsof which are incorporated herein by reference. Likewise, the processormay interpret the signals detected by the antennas using the codingtechniques described in the '297 patent.

The measurement of the “z”-axis field, described in the '297 patent, isnot directly available, as no single antenna 30, 32, 34, 36 receives allof the “z” field as in that application. However, the “z”-field can bereadily calculated from computing the component vectors in the “z”direction.

Therefore, assuming the tracker 12 is over the beacon 24 and orientedproperly, each pair of antennas will receive a component of the “z”field. The vector sum can be calculated using the components calculated,giving the total field for the purposes of the measurements disclosed inCole and incorporated herein.

In addition, amplitude modulation, that is, varying the signal strengthof the beacon signal, may be used to interpret the signals received bythe antenna. Alternatively, or in conjunction with amplitude modulation,phase modulation may also be used with the antenna arrangement 10 tointerpret the antenna signals.

In operation, none of the antennas 30, 32, 34, and 36 will approach thenoise floor. Rather, ambient noise is present in all of the antennas 30,32, 34, and 36. However, since the antenna arrangement 10 focuses onbalancing signals rather than minimizing signals, the present noiseinterferes less with finding the “null” points.

Changes may be made in the construction, operation and arrangement ofthe various parts, elements, steps and procedures described hereinwithout departing from the spirit and scope of the invention asdescribed in the following claims.

The invention claimed is:
 1. An above-ground antenna arrangementcomprising: a first conductive loop; a second conductive loop,surrounded by the first conductive loop; a third conductive loop; and afourth conductive loop, surrounded by the third conductive loop; inwhich each of the first, second, third and fourth conductive loops areplanar and define separate planes; and in which each of the first,second, third and fourth conductive loops have a common centerpoint; andin which the planes of the first and second conductive loops intersectat a first line and the planes of the third and fourth conductive loopsintersect at a second line; wherein the first line and second line arein spaced relationship.
 2. A system, comprising: the antenna arrangementof claim 1; and a beacon disposed at a below-ground location andgenerating a dipole magnetic field about its length; wherein the antennaarrangement is receiving the field generated by the beacon.
 3. Thesystem of claim 2 further comprising: a horizontal directional drill; adrill string coupled to the horizontal directional drill having a firstend and a second end; in which the horizontal directional drill iscoupled to the drill string at the first end; and in which the beacon isdisposed at the second end of the drill string.
 4. A method comprising:using the system of claim 3, advancing the drill string and beacon; andthereafter, moving the antenna arrangement to a position on a surface ofthe ground where the strengths of the magnetic dipole field detected bythe first and second conductive loops are equal, and where the strengthsof the magnetic dipole field detected by the third and fourth magneticloops are equal.
 5. The method of claim 4 further comprising orientingthe antenna arrangement such that the planes in which the third andfourth conductive loops are situated are parallel to the length of thebeacon.
 6. An antenna assembly having a first, a second, a third, and afourth conductive loop, each of the first, second, third and fourthconductive loops situated within a single plane and having a center, inwhich the centers of the four loops coincide and which none of theplanes of the four loops coincide, wherein the first and secondconductive loop intersect at a first reference line and the third andfourth conductive loop intersect at a second reference line, the firstand second reference lines being non-parallel.
 7. The antenna assemblyof claim 6 in which the planes of the first and second conductive loopsare orthogonal.
 8. The antenna assembly of claim 7 which the planes ofthe third and fourth conductive loops are orthogonal.
 9. An antennaassembly, comprising: a first conductive loop situated in a first planeand defining a first normal vector, normal to the first plane andextending through a centerpoint of the first conductive loop; a secondconductive loop situated in a second plane and defining a second normalvector, normal to the second plane and extending through a centerpointof the second conductive loop; a third conductive loop situated in athird plane and defining a third normal vector, normal to the thirdplane and extending through a centerpoint of the third conductive loop;a fourth conductive loop situated in a fourth plane and defining afourth normal vector, normal to the fourth plane and extending through acenterpoint of the fourth conductive loop; in which the centerpoints ofthe first, second, third and fourth conductive loops are coincident; andin which the first, second, third and fourth normal vectors are distinctand not co-planar.
 10. The antenna assembly of claim 9 in which each ofthe first, second, third and fourth conductive loops are supported on aprinted circuit board.
 11. The antenna assembly of claim 9 in which eachof the first, second, third and fourth conductive loops comprise coilsof conductive wire.
 12. The antenna assembly of claim 9 in which: thefirst normal vector and the second normal vector are perpendicular; andthe third normal vector and the fourth normal vector are perpendicular.13. A method of using the antenna assembly of claim 5, comprising:generating a dipole magnetic field at an underground location to whichthe antenna assembly is responsive; moving the antenna assembly withinthe magnetic field to an above ground location where the field strengthsdetected by the first and second conductive loops are equal and thefield strengths detected by the third and fourth conductive loops areequal.
 14. The method of claim 13 further comprising: after moving theantenna assembly to the above ground location, detecting a magnitude ofthe magnetic dipole field at the above ground location.
 15. An antenna,comprising: a first conductive loop having a center; a second conductiveloop, electrically independent of the first conductive loop, the secondconductive loop having a center that coincides with the center of thefirst conductive loop; a third conductive loop having a center, thethird conductive loop electrically independent of the first and secondconductive loops; and a fourth conductive loop, electrically independentof the first, second and third conductive loops, and having a centerthat coincides with the center of the third conductive loop; in whichthe first conductive loop surrounds the second conductive loop; in whichthe third conductive loop surrounds the fourth conductive loop; in whichthe planes of the first and second loops intersect at a first line; inwhich the planes of the third and fourth loops intersect at a secondline; and in which the first line and the second line are perpendicular.16. The antenna of claim 15 in which the centers of the first, second,third and fourth conductive loops coincide.
 17. The antenna of claim 15in which each loop is characterized by a plane that contains itsperimeter, and in which the planes of the first and second loops areorthogonal.
 18. The antenna of claim 17 in which the planes of the thirdand fourth loops are orthogonal.
 19. The antenna of claim 15 in whichthe second conductive loop surrounds the third conductive loop.
 20. Theantenna arrangement of claim 1, in which: the antenna array is situatedrelative to a reference plane; the plane of the first loop is situatedat an equal and opposite angle relative to the reference plane as theplane of the second loop; and the plane of the third loop is situated atan equal and opposite angle relative to the reference plane as the planeof the fourth loop.
 21. The antenna arrangement of claim 20 in which thereference plane is horizontally oriented.